A polymer electrolyte fuel cell generates electricity and heat simultaneously by electrochemically reacting a fuel gas containing hydrogen and an oxidant gas containing oxygen such as air. Typically, the polymer electrolyte fuel cell comprises a proton conductive polymer electrolyte membrane, electrodes and conductive separators. The electrodes each comprise a combination of a diffusion layer and a catalyst reaction layer. The catalyst reaction layer, which is composed mainly of a carbon powder carrying a platinum group metal catalyst thereon, is formed on each surface of the polymer electrolyte membrane that selectively transports hydrogen ions. On the outer surface of the catalyst reaction layer is formed the diffusion layer having electron conductivity and gas permeability for fuel gas or oxidant gas.
In order to prevent supplied fuel gas and oxidant gas from leaking out and to prevent them from mixing with each other, gas sealants or gaskets are arranged on the periphery of the electrodes with the polymer electrolyte membrane sandwiched therebetween. The gas sealants or gaskets are combined in advance with the electrodes and the polymer electrolyte membrane. This is called as an MEA (membrane electrode assembly).
On the outer surfaces of the MEA are arranged the conductive separators for mechanically fixing the MEA and electrochemically connecting adjacent MEAs in series. On the surface of each conductive separator facing the MEA is formed a gas flow channel for supplying a fuel gas or oxidant gas to an electrode and exhausting a produced gas or excess gas. The gas flow channel may be formed by providing a separate member to the separator. Usually, a groove or rib is formed on the surface of the separator by means of cutting or pressing the separator surface to serve as a gas flow channel.
In order to supply a fuel gas or oxidant gas to the groove, a pipe for supplying a fuel gas or oxidant gas must be branched into individual separators. Further, a plumbing jig is necessary in which the branched pipes are connected to the grooves on the separators. Such a jig is called as an “external manifolds”. The conduit through which the fuel gas or oxidant gas passes is called as an “manifold aperture”.
There is another type of manifold called “internal manifold”, which has a simpler structure. In an internal manifold type, a through hole is formed through a cell stack comprising an MEA and separators each having a gas flow channel, in the stacking direction thereof, and is used as a manifold aperture. Around each through hole in the MEA and the separators, a rib is formed or an O-ring is arranged therearound, to provide sufficient sealing. The manifold aperture is connected to a pipe for supplying a fuel gas or oxidant gas so that a fuel gas or oxidant gas is supplied directly from the aperture.
Since a fuel cell generates heat during its operation, it should be cooled down with cooling water or the like to maintain the cell at an appropriate temperature. Normally, a cooling unit for flowing cooling water is provided between the separators for every 1 to 3 unit cells. The cooling unit is usually a cooling water flow channel formed on the back surface of the separator.
In a typical fuel cell, 100 to 200 unit cells, each comprising an MEA, conductive separators and a cooling unit, are stacked alternately to form a cell stack, which is fixed with bolts from both ends.
In the internal manifold type stack, the gas sealants or gaskets disposed sandwiching the polymer electrolyte membrane may sometimes enter the ends of the gas flow channel, which are connected to the manifold apertures, respectively. As a consequence, a gap is created between the separators and the MEA, through which the gas leaks. In order to prevent the gas sealants or gaskets from entering the ends of the gas flow channel, for example, Japanese Laid-Open Patent Publication No. Hei 9-35726 proposes to form a tunnel structure by covering each end of gas flow channel, which is connected to corresponding manifold aperture, with a flat cover plate.
In the above separator structure, the ends of gas flow channel are covered with cover plates, respectively, but the cover plates are not fixed. This creates loss of time in the production of a cell stack. For example, it will be difficult to assemble a cell stack with the surface of the separator having a gas flow channel formed thereon facing downward.
In order to cope with this problem, attempts were made to fix the separator and the cover plate with an adhesive. However, the use of an adhesive may cause displacement beyond acceptable dimensional tolerance. Moreover, there is a concern that the plasticizer contained in the adhesive and unreacted lower molecular components might leach out, causing damage to the MEA.